This invention relates to pipelines, and particularly to an oil transportation pipeline especially adapted for use in transporting petroleum over long distances in an Arctic environment.
Because of the large petroleum resources available at the north slope of Alaska, a great deal of research and study has been undertaken in recent years in order to determine whether it would be feasible to construct an oil pipeline extending from the north slope of Alaska to a port on the south coast. One proposal, which has received a great deal of attention, relates to the construction of a 789 mile long pipeline extending from a point south of Prudhoe Bay on the north slope to Port Valdez. Numerous technical problems have to be overcome in the implementation of such a proposal. Undoubtedly, the greatest problem is the effect of the proposed pipeline on the Arctic environment, and the magnitude of the problem may be appreciated by reference to Final Environmental Impact Statement, Proposed Trans-Alaska Pipeline, Government Printing Office, 1972. Various other technical problems relating to construction and maintenance of the pipeline also exist.
In general, there are two types of pipeline construction, each of which has its own inherent problems.
Above-ground construction would result in a considerable disturbance to the Arctic wildlife. For example, an above-ground pipeline would constitute a barrier, preventing the normal movement of caribou, moose, sheep, musk-ox, and bison. It is believed that an above-ground pipeline would be particularly detrimental to caribou, since they are characteristically migratory, and their well-being depends upon movement to traditional calving and seasonal grazing areas. Although moose, sheep, musk-oxen and bison are not considered migratory in the same sense as caribou, they do undergo seasonal movements of a moderately extensive nature. Obstruction of the seasonal movements of these animals would reduce the utilization of their habitat and perhaps isolate essential components of their ranges.
The construction of an above-ground pipeline would also involve extremely high construction and maintenance costs. For example, millions of cubic yards of gravel and crushed rock would be required to construct a support for the pipeline. The obtaining and transportation of gravel and crushed rock would result in extremely high costs. Similarly, high costs would be involved in the construction of concrete pads, steel plates and sliding cradles to support the pipeline. In an above-ground pipeline, certain parts of the line would have to be buried, and this requirement would give rise to particularly complex and costly problems.
The maintenance of an above-ground pipeline would include the costs of repair of lightning damage, snow removal, the cost of repairing damage caused by vandalism and sabotage, and the cost of surveillance.
A buried pipeline, on the other hand, has its own inherent problems, perhaps the most significant of which is the effect of the pipeline temperature on the permafrost. Permafrost is frozen soil, much of which contains large quantities of frozen water. Its strength is dependent upon the frozen condition of its water content. Thus, if melting occurs, the permafrost may no longer provide support. Oil, on the other hand, only flows efficiently at relatively high temperatures, its viscosity varying inversely with temperature. Furthermore, at high flow rates, oil temperature may actually increase in the pipeline. An ordinary pipeline, buried in permafrost, would transmit heat to the permafrost, and possibly cause its own destruction by melting away its support. Even if adequate support remained, the application of heat by the pipeline to the permafrost over long periods of time would cause a pool of water to collect which would continue to grow into a large pond or "thaw bulb", possibly causing a great deal of damage to the pipeline and the environment.
Other problems inherent in buried oil pipeline are the difficulty in accommodating expansion and contraction which result from temperature changes, and the difficulty of detecting and repairing oil leaks.
A problem inherent in oil pipelines, whether above-ground or buried, is the limitation on throughput imposed by the maximum pressure-withstanding capacity of the pipeline.
The principal object of this invention is to provide a buried pipeline in which the aforementioned problems are overcome.
Briefly, the pipeline comprises an inner oil-carrying conduit surrounded by a series of annular evacuated chambers, the chambers being in turn surrounded by a pair of coaxial conduits in which a liquid coolant is recirculated under pressure.
The temperature of the liquid coolant is controlled so that the pipeline does not alter the condition of the permafrost in which it is buried even though the oil being transported in the pipeline may be at a very high temperature.
The control of the temperature of the coolant-carrying conduits limits their expansion and contraction, and expansion joints are provided in the pipeline to permit longitudinal expansion and contraction of the oil-carrying conduit and the surrounding vacuum sections.
The recirculating coolant is a transparent liquid which facilitates leak detection by photoelectric means. If oil in the central conduit leaks into the coolant system, photoelectric detectors, responding to the reduced transparency of the coolant, register the existence and approximate location of the leak. The photoelectric system may be used as a back-up for other leak detecting devices which are explained hereinafter in greater detail.
When oil leaks into the coolant system in large quantities, it may be recovered and returned to the oil conduit by oil recovery means located at the coolant pumping stations which are located at intervals along the pipeline.
Finally, the pressure of the coolant in the coolant conduits imposes an inward pressure on the exterior of the oil transporting conduit through the vacuum section structure, permitting the oil to reach higher pressures, and therefore higher flow rates, in the oil conduit, than in a conduit of equal size in a conventional single conduit pipeline.
Other objects will be apparent from the following detailed description when read in conjunction with the drawings.